This invention relates to reduction of distortion in audio systems and particularly to reduction of distortion in amplifiers from heretofore unrecognized causes.
In the co-pending application referred to above, the previously unrecognized effects of time displacement distortions on audio sound quality in speaker systems was disclosed. Such distortions are, in general, due to radio freqency (RF) energy pickup and stored energy in non-linear reactive elements, both of which contribute to increasing the so-called "noise floor" of the system. An increase in the noise floor reduces the system dynamic signal range capability such that sharp differentiations between "periods of sound" and "periods of silence" are lost. Anything that increases the system noise floor is detrimental to good quality audio reproduction. These effects may only be discernible by discriminating listeners with high quality audio equipment. Time displacement distortions in speaker systems incorporating crossover networks, for dividing the frequency spectrum among a plurality of drivers, were especially dealt with. In particular, control of the specific driver back EMF energy was accomplished with back EMF dissipating resistors.
The present invention is directed to improvements in the sound of audio amplifier circuits by reducing energy storage by shock excited reactive circuit elements. This energy is believed to modulate the various junctions of transistor amplifying devices, generally through the power supply and ground connections to the devices. The shock excitation of reactive elements such as capacitors and inductors is believed to result from the transient and unsymmetrical nature of audio signals which produce a continuum of undesirable transient signal currents. While these undesirable signals across the reactive elements may be small in magnitude, on the order of a few millivolts or less, they are not insignificant because the signal voltage required across the input terminals of transistors is in the same range. Further, the distortion effects produced are cumulative. Even though each individual distortion may be miniscule, their combined effect is quite noticeable. Circuits constructed in accordance with prior art techniques suffer in varying degrees from these distortion effects and, it is believed, that all such circuits can benefit from the application of the prinicples of the invention.
Since the nature of sound is periods of energy separated by periods of silence, the importance of accurately reproducing the distinction between these two states is immediately apparent. The distortion produced by shock excitation of capacitors and inductors in the audio signal path partially "fills in" the periods of silence and is quite noticeable in high quality audio systems. These time displacement distortions are similar to the frequency modulation distortion produced by speed variations in tape recorders and turntables. A difference is that the time displacement distortion is a function of the ability of the audio signal to shock excite the reactive circuit element and is therefore dependent upon the characteristics of the signal itself. In particular, the distortion is related to the amount of transient energy in the signal. The prior art has not recognized this form of distortion.
It has been found that time displacement distortions can be reduced significantly by providing a resistive energy-dissipating path for each reactive component. One would think that to be effective the shunt resistors would have to be low in value, particularly when used with large values of capacitance and inductance. Surprisingly however, it has been discovered that resistor values in the tens of thousands of ohms can provide the desired distortion reduction. Thus, the overall efficiency of the amplifier is not compromised. In the prior art, shunt resistors across reactive components are only found across capacitors that are capable of dangerous levels of energy storage and are designed to discharge such capacitors quickly to reduce that danger. The high value shunt resistors of the present invention are thus quite different from the shunt resistors used in the prior art. The energy dissipating resistors of the invention are also used in discharge paths for capacitors having minimal energy storage and no capability for dangerous energy levels, as well as across inductive elements, such as transformers.